Faraday isolators have long been used for the purpose of allowing a light beam to be transmitted in one direction with low loss while highly attenuating a beam traveling through it in the opposite direction. An important component of the optical isolator is a Faraday rotator, which exhibits the Faraday effect. The Faraday effect rotates the plane of polarization of linearly polarized light through an angle. The magnitude of the angle is dependent upon the length of the Faraday rotator and the magnitude of the magnetic field it imposes. Accordingly, to attain a predetermined angular rotation, with a fixed magnetic field, the length of the Faraday rotator must be appropriately adjusted. Since the Faraday effect is reversible, the light can be reflected back through the Faraday rotator after the first traversal, doubling the rotation of the polarization angle.
When dealing with relatively low energy optical beams, a Faraday rotator experiences a modest heating effect. This heating effect results in thermal gradients in the Faraday rotator, producing a "smearing" of the polarization angle rotation.
When a Faraday rotator is used with high optical powers (i.e., over 10 watts), the resulting degradation in optical beam quality due to Faraday rotator heating, becomes unacceptable.
In the past, Faraday rotators have been cooled. However, since most Faraday rotators are transmissive, the only cooling possible was that which could be accomplished by cooling the edges of the Faraday rotator. For two reasons this led to undesirable effects. One reason is that only a small percentage of the total area of a Faraday rotator is located on its edges. A second reason is that heat removal from the edges of a Faraday rotator still produces the undesirable thermal gradients in a transverse direction.
It is therefore desirable to have a Faraday isolator which is capable of handling high energy light beams without experiencing significant beam degradation due to heating effects.